Forget about run off pH in coco - I'd even say forget about input pH as well, but if you want to check it going in then thats ok. Make sure its between 6.0 - 6.5 and if it falls between those figures forget about it.
As for your run off:
PPM going up, run off water volume going down - add more irrigations - pot is drying out between feeds and causing a concentration of salt in the medium
PPM going up, run off water volume stays the same - add less nutrients - plants are dashing the excess feed
PPM going down, run off water volume the same - add more feed. - plants are consuming feed quicker than they are drinking
PPM stays the same, run off water volume is the same/adquate - you've nailed it dont change anything - plants are drinking and consuming at the same rate, perfection
Coco is easy, don't complicate it